A “mobile ad hoc network” (referred to as a MANET herein) is a collection of wireless nodes (mobile hosts) that can dynamically be set up anywhere and anytime without using any pre-existing network infrastructure. MANET is a type of packet switching network wherein each mobile device is an autonomous node, which may function as both a host and a router. In other words, besides the basic processing ability as a host, the mobile devices can also perform switching functions as a router forwarding packets from other nodes. MANET is an autonomous system in which each mobile device is free to move randomly and independently in any direction, and will therefore change its wireless links to other devices frequently. Since the nodes are mobile, the network topology may change rapidly and unpredictably and the connectivity among the terminals may vary with time. Thus, the mobile nodes in the network should adapt to the traffic and propagation conditions and dynamically establish routing among themselves as they move about, forming their own network on the fly. The wireless topology of a mobile ad hoc network may change rapidly and unpredictably. The structure of a mobile ad hoc network is not fixed, i.e. nodes can be added to or removed from the network while the network is operational without causing irreversible failures. Such a self-configuring network may operate in a standalone fashion, or may be connected to the larger Internet.
Since the topology of the network is constantly changing, the issue of routing packets between any pair of nodes becomes a challenging task. Basic types of ad hoc routing algorithms can be single-hop and multihop, based on different link layer attributes and routing protocols. Single-hop network is simpler than multihop network in terms of structure and implementation, with the cost of lesser functionality and applicability. In the multihop scenario, when delivering data packets from a source node to its destination node out of the direct wireless transmission range, the packets should be forwarded via one or more intermediate nodes. However, a single-hop network sends data directly from a source node to a destination node. In practice, nearby nodes can communicate directly by exploiting a single-hop wireless technology (e.g., Bluetooth, 802.11, etc.), while nodes that are not directly connected communicate by forwarding their traffic via a sequence of intermediate nodes. The forwarding is not necessarily “on-the-fly”. Intermediate nodes may store the messages when no forwarding opportunity exists (e.g., no other nodes are in the transmission range, or neighbors are not suitable for that communication), and exploit any contact opportunity with other mobile devices to forward the data toward the destination. The failure of a set of links or nodes in the underlying network can cause the network to break apart into two or more components or clusters. As a result of this, nodes within a cluster can communicate each other, but communication between the nodes in different clusters is not possible. Very often the network will partition and remerge, affecting the performance of routing protocols.
Examples of such networks vary from radio-linked sensors distributed like seed by air drop, to the behavior of satellites in random orbits, to automotive applications in which cars and traffic lights are communicating nodes, to military applications such as battlefield communications among soldiers, vehicle-mounted devices, and consumer devices at home.
Among multi-hop ad hoc networks, wireless sensor networks have a special role. A sensor network is composed of a large number of small sensor nodes, which are typically densely (and possibly randomly) deployed inside the area in which a phenomenon is being monitored. Wireless multi-hop ad hoc networking techniques constitute the basis for sensor networks, too. However, the special constraints imposed by the unique characteristics of sensing devices, and by the application requirements, make the solutions designed for multi-hop wireless networks (generally) not suitable for sensor networks. Sensor networks produce a shift in the networking concept from a node-centric to a data-centric view. The aim of a sensor network is to collect information about events occurring in the sensor field rather than supporting the communications between users' devices.
Moreover, power management is an important issue in the design of all light-weight mobile devices with low CPU processing capability, small memory size, and low power storage. Such devices need optimized algorithms and mechanisms that implement the computing and communicating functions. Also sensor network nodes utilize on-board batteries with limited energy that cannot be replenished in most application scenarios. Thus, the communication-related functions should be optimized in the power consumption point of view. Conservation of power and power-aware routing must be taken into consideration.
Many wireless devices support power-saving modes in which the radio of a mobile node only needs to be awake periodically. The period of time, when a mobile node is in a sleep mode, is called a sleep period, and the period of time when, a mobile node is in an active or awake mode is called an awake period herein. Typically, at least the radio communication parts, i.e. a radio transmitter and radio receiver, of a mobile node are turned off for the sleep period, but also other parts of the node, such as a processor, may assume a power-saving state. During the awake period, at least a radio receiver of a mobile node is turned on, and optionally a transmitter, if transmission is necessary. The lower power consumption is achieved, when a mobile node has its radio turned off most of the time, and the radio is turned on only during short awake periods for communication with other nodes. This enables a significant saving in the power consumption compared to leaving a device fully on all the time. In order to enable communication, nodes should wake up synchronously at the same time. The synchronous wakeup operation is easily achieved in a master-slave configuration, when mobile nodes are timed from a network infrastructure, such as from a fixed station. However, in a multi-hop ad hoc network there normally is no master node available for timing the sleep and awake periods of other “slave” nodes. The master node configuration is also very vulnerable in a multi-hop mobile ad hoc network: if a master node is removed or fails, all slaves would lose their synchronism. In a “no master” configuration, mobile nodes wake up more or less asynchronously, and data communication may fail, or at least there will be a long propagation delay, when a message propagates through a multi-hop network: a first radio wakes up, waits until a neighboring second node wakes up, and sends a message to the second node, then the second node waits until a neighboring third node wakes up, etc.